Thin films of GaNBi alloys with up to 12.5 at.% Bi were grown on sapphire using low-temperature molecular beam epitaxy. The low growth temperature and incorporation of Bi resulted in a morphology of nanocrystallites embedded in an amorphous matrix. The composition and optical absorption shift were found to depend strongly on the III:V ratio controlled by the Ga flux during growth. Increasing the incorporation of Bi resulted in an increase in conductivity of almost five orders of magnitude to 144 Ω-cm−1. Holes were determined to be the majority charge carriers indicating that the conductivity most likely results from a GaNBi-related phase. Soft x-ray emission and x-ray absorption spectroscopies were used to probe the modification of the nitrogen partial density of states due to Bi. The valence band edge was found to shift abruptly to the midgap position of GaN, whereas the conduction band edge shifted more gradually.

We have studied the low-temperature growth of GaNAs layers on sapphire substrates by plasma-assisted molecular beam epitaxy. We have succeeded in achieving GaN1-xAsx alloys over a large composition range by growing the films at temperature much below the normal GaN growth temperatures with increasing the As2 flux as well as Ga:N flux ratio. We found that the alloys with high As content x>0.1 are amorphous. Optical absorption measurements reveal a continuous gradual decrease of band gap from ˜3.4 eV to ˜1.4 eV with increasing As content. The energy gap reaches its minimum of ˜1.4 eV at the x˜0.6-0.7. For amorphous GaAsN alloys with x<0.3 the composition dependence of the band gap follows the prediction of the band anticrossing model developed for dilute alloys. This suggests that the amorphous GaN1-xAsx alloys have short-range ordering that resembles random crystalline GaN1-xAsx alloys. Such amorphous GaN1-xAsx alloys with tunable electronic structure may be useful as photoanodes in photo-electrochemical cells for hydrogen production.

Transmission electron microscopy (TEM) and atomic force microscopy (AFM) have been used to analyse the core structure of dislocations in GaN grown by molecular beam epitaxy (MBE) as a function of the gallium to nitrogen ratio. Ga-rich samples had a much smoother morphology; TEM observations showed that amorphous deposits decorated some dislocations and occasional surface pits, but weak beam and end-on imaging suggested that, away from the growth surface, dislocations of all types had closed core structures, in contrast to previous observations (Hsu et al, Appl. Phys. Lett. 78, 3980 (2001), Baines et al, Mat. Res. Soc. Symp. Proc. 743, L2.5 (2003)). Ga-poor samples were found to have much rougher surfaces; dislocations were often at the centers of deep surface pits but were observed to be undecorated and to have closed core structures. It is concluded that in growth under Ga-rich conditions, decoration of dislocation cores depends on the accumulation of Ga at surface pits, rather than being a fundamental property of dislocation formation.